1. Field of the Invention
The invention relates to IR sensors incorporating scanned staggered element linear arrays of IR detectors maintained at cryogenic temperatures and more particularly to the provision of improved cold shielding for such arrays.
2. Prior Art
Effective cold shields are required for infrared arrays to limit the unfocused illumination incident on the array. With line arrays, the simplest approach is to locate the IR array in a cold shielded container, to hold the unfocused radiation to a negligible level compared with the focused radiation from the scene. One known approach is to pass the focused radiation from the optics through a long narrow slit. When the optics are of low speed, i.e. high F/#, this simple approach is very effective. When the optics are required to be "fast", i.e. of low F/#, the slit must be made quite wide and then considerable unfocused background illumination can fall on the IR detector, which will seriously degrade the target contrast, and in consequence, the dynamic range and ultimate sensitivity of the system.
When a fast optical system is employed to obtain the highest IR system sensitivity in a scanning infrared sensor it is difficult to maintain high cold shielding efficiency. Cold shielding efficiency may be defined as the ratio of focused to unfocused radiation incident on the IR detector. The problem with wide angle systems is that they present large areas of relatively high temperature IR radiating surfaces to the IR sensors. Such surfaces include the internal barrels of the optics and to a lesser degree the optical elements themselves. At the limit, the unfocused background of individual photo detectors in an IR detector array may have an angular field of view equal to a full hemisphere. The object for the ideal cold shield is to prevent IR background radiation lying outside the focused angular field of view from reaching the individual photodetectors.
A known cold shield approach operates at the pixel level for line arrays. The shield is formed making use of the anisotropic etching properties of silicon. An anisotropic etch is used to form a series of apertures in a thin silicon plate which is positioned over the line array to limit the acceptance field of each individual pixel. This known method of forming pixel level cold shield apertures is limited in constraining the pixel angular field in the optical scan direction due to the non-rectangular nature of a silicon preferentially etched aperture. The pair of end walls of such etched slots are not normal to the almost perfectly vertically defined pair of side walls.
The more critical limitation of the preferentially etched silicon apertures of the known approach is in application to arrays with over-lapped pixels in a staggered element linear array. This array type constitutes the majority of linear IR array applications of interest today. In a simple non-staggered element linear array, the enlarged non-normal end walls of the preferentially etched apertures cause only the minor problem noted above, but in the staggered element linear array, the end effects of one aperture spoil the overlapped apertures of neighboring pixels, thus greatly compromising the IR background restriction gained in application to a simple non
staggered linear array.
Even if the cold shield using preferentially etched silicon could achieve perpendicular end walls on all four sides of the aperture, the geometry of the overlapped pixel staggered element array precludes effective use. In a staggered element line array the pixels lie in closely spaced adjacent columns, the pixels in one column being off-set by half the pixel interval from the pixels in the other column. Under these circumstances, perfectly rectangular apertures for the pixels in one column would enlarge the angular field of view and so worsen the cold shielding for pixels in the other column.